Various systems and methods of testing have been developed that shut a valve in the supply pipe to a contained fluid system and measure the decay of a signal detected by a pressure sensor over time, to determine the rate at which a system is leaking, or gas is flowing. This typically requires a full range pressure sensor to avoid over-pressurizing the sensor and to allow moderate to larger flows to be more accurately detected, and this makes such systems inherently less sensitive.
Systems have also been developed using various flow amplification measurement methods. Such methods include using a Venturi with a differential, pressure transducer, or a smaller bypass pipe through which all flow is directed. These arrangements amplify the dynamic pressure signal by increasing the velocity of flow in the smaller diameter section. Since dynamic pressure, or equal drop in static pressure, varies as the square of velocity, the signal is amplified by the greater velocity in the contracted section, and the sensitivity increased. In the case of a Venturi, or any other differential flow measuring means, a more sensitive flow measurement means can be used because it does not have to withstand the full static pressure range, but only the dynamic pressure, which is equal to the drop in static pressure, caused by flow.
Typically flow measuring devices, and particularly, pressure transducer devices, have a temperature compensation signal conditioner, and the more accurate the compensation, the higher the price of the transducer. However, even very expensive sensors do not have sufficient signal resolution for measurement of very small flows. Such sensors are typically limited to a small percentage of maximum pressure range for reasonable accuracy. Consider a system where the maximum flow in a pipe is 5 ft/sec, amplified to 10 ft/sec by a Venturi contraction. The pressure is proportional to velocity squared, or 100. If the flow is 0.5 ft/sec the pressure is proportional to 1, or about the range of accuracy of a sophisticated sensor with fullscale set for the dynamic pressure range from a flow of 5 ft/sec. Suppose the velocity is 0.05 ft/sec, which would mean a sizeable leak, the pressure is proportional to 0.01, which is one thousandth of the transducer range. The transducer signal from such pressure change is less than the random pressure fluctuations in the pipe, and is well outside the manufacturer's tolerance, hysteresis and temperature variation of the pressure sensor, which typically exceeds 1%. The measurements can no longer be relied upon, and will be unable to provide a meaningful reading for lesser fluid flows from still smaller leaks.
In considering a leak detection system for a residential gas system, both price and accuracy are important. It is also necessary not to interfere with the normal use of the gas system, nor to have the device or test procedure extinguish any pilot light that may exist in the gas system. A small leak must be detected on top of the pilot light flow without extinguishing the pilot light, and the pilot light flow is already below the noise and the component manufacturer's tolerance level of the most sophisticated flow measurement devices, even when they are mounted in flow amplification devices as described above.
The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.